Wireless power transmission technology that transmits electric power between two points in space without use of a power transmission cable includes a magnetic resonance power transmission technology that uses magnetic resonance to transmit power. In the magnetic resonance power transmission system, a coil (i.e., LC resonance coil) having a resonance function based on capacitor and inductor is provided on each of the transmission side and the reception side. The LC resonance coil provided on the transmission side and the LC resonance coil provided on the reception side have a common resonance frequency. Electric power is transmitted from the transmission side to the reception side via a high-frequency alternating-current magnetic field that oscillates at this resonance frequency.
The magnetic resonance power transmission system allows longer power transmission distance (i.e., distance in the direction perpendicular to the coil plane) and a wider margin for the position of a power receiving side (i.e., greater tolerance to a displacement in the direction parallel to the coil plane) than the wireless power transmission system that uses electromagnetic induction without utilizing resonance. Further, highly efficient power transmission is achieved between coils having different sizes.
Wireless electric charge systems that use electromagnetic induction without utilizing resonance are used in chargers for rechargeable batteries, and have already been commercially available. Such chargers require exact positional alignment between the power-transmission-side coil and the power-reception-side coil in order to transmit power through electromagnetic induction. For example, a rechargeable battery may be placed with respect to a recess of a charger such that the negative pole of the rechargeable battery faces the bottom surface of the recess, thereby achieving exact positional alignment. There may be a case, however, where a plurality of batteries may need to be charged. Aligning the individual batteries with respect to the respective recesses of a charger in the manner as described above is quite cumbersome.
The use of magnetic resonance in place of electromagnetic induction allows efficient wireless electric charge without exact positional alignment relative to a charger. In the case of charging a plurality of batteries at once, the use of electromagnetic induction may require that a plurality of power transmitting coils be provided for the respective batteries. The use of magnetic resonance, on the other hand, allows a single large power transmitting coil to efficiently transmit power to a plurality of power receiving coils for electric charge.
A typical shape of a rechargeable battery is a cylinder. In order to secure a large power receiving area size, it is preferable to provide the power receiving coil on the side surface of the rechargeable battery. Further, in order to charge a plurality of rechargeable batteries without requiring these batteries to be placed in specific position and/or at specific spatial locations, a large power transmitting coil may be disposed across the entirety of a charger platform that is placed in a horizontal position. Rechargeable batteries may then be laid horizontally on the surface of the charger platform. In this arrangement, efficient power reception may be achieved when the power transmitting coil embedded in the charger platform and the power receiving coil embedded in a rechargeable battery substantially face each other. With a power receiving coil situated at a specific position on the lateral surface of a rechargeable battery, however, the fact that cylindrical rechargeable battery is easy to rotate makes it difficult to keep the battery still in such a position that the power receiving coil and the power transmitting coil face each other.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2010-193701    [Patent Document 2] Japanese Laid-open Patent Publication No. 2004-260917